The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically to a semiconductor device including an inductor and a method for fabricating the semiconductor device.
LSIs which process signals of radio-frequencies of an about 2 GHz band effectively include inductors formed in the semiconductor chips so as to process signals of a wide frequency band.
FIG. 12 is a conceptual view of a conventional semiconductor device including an inductor.
A MOSFET, etc. not shown are formed on a p type silicon substrate 110. An insulation film 128 is formed on the silicon substrate 110 with the MOSFET, etc. formed on. An inductor 132b is formed on the insulation film 128. Bonding wires 138 are connected to the inductor 132b. 
Thus, the conventional semiconductor device with the inductor 132b is formed.
In the semiconductor device shown in FIG. 12 large eddy current as shown in FIG. 12 is generated in the silicon substrate 110 below the inductor 132b. An eddy current loss lowers Q of the inductor 132b by 3 to 8.
Q of the inductor is an index which indicates low-loss and sharp resonance characteristics. Recently, Q is required to be above 20.
To improve Q of the inductor, the semiconductor device as shown in FIG. 13 is proposed. FIG. 13 is a conceptual view of the proposed semiconductor device.
As shown in FIG. 13, a plurality of trenches 112 are formed in a p type silicon substrate 110. A highly resistive layer 114 of polysilicon is buried in the trenches 112. An insulation film 128 is formed on the silicon substrate 110 with the highly resistive layer 114 buried in. An inductor 132b is formed on the insulation film 128.
In such semiconductor device, the highly resistive layer 114 is buried in the silicon substrate 110 to thereby increase a resistivity below the inductor 132b, whereby an eddy current loss can be smaller, and Q of the inductor 132b can be improved.
However, in the proposed semiconductor device shown in FIG. 13, it is necessary to form the trenches 112 in the silicon substrate 110, and the highly-resistive layer 114 is buried in the trenches 112. The proposed semiconductor device has a larger number of fabrication steps by about 10 steps in comparison with the semiconductor device shown in FIG. 12. Such increase of fabrication steps leads to higher costs of the semiconductor device. A technique for increasing Q of the inductor without burying the highly resistive layer in the silicon substrate has been required.
An object of the present invention is to provide a semiconductor device and a method for fabricating the semiconductor device which can form an inductor of high Q by simple steps.
According to one aspect of the present invention, there is provided a semiconductor device comprising: a silicon substrate of a resistivity above or equal to 800 xcexa9xc2x7cm and an oxygen concentration under or equal to 5xc3x971017 cmxe2x88x923; and an inductor formed in the silicon substrate. A concentration of oxygen contained in the silicon substrate is set to be low, whereby the silicon substrate is less vulnerable to thermal donor effect, and even in a case that a silicon substrate of high resistivity is used, a semiconductor device which suppresses conversion of a conduction type of the silicon substrate while having an inductance of high Q. It is not necessary to bury a highly resistive layer in the silicon substrate, whereby a semiconductor device having an inductance of high Q can be fabricated by simple fabrication steps, which contributes to cost reduction of the semiconductor device.
According to another aspect of the present invention, there is provided a method for fabricating a semiconductor device comprising: the step of forming an inductor in a silicon substrate of a resistivity above or equal to 800 xcexa9xc2x7cm and an oxygen concentration under or equal to 5xc3x971017 cmxe2x88x923. A concentration of oxygen contained in the silicon substrate is set to be low, whereby the silicon substrate is less vulnerable to thermal donor effect, and even in a case that a silicon substrate of high resistivity is used, a semiconductor device which suppresses conversion of a conduction type of the silicon substrate while having an inductance of high Q. It is not necessary to bury a highly resistive layer in the silicon substrate, whereby a semiconductor device having an inductance of high Q can be fabricated by simple fabrication steps, which contributes to cost reduction of the semiconductor device.
As described above, according to the present invention, a concentration of oxygen contained in a silicon substrate is set low, whereby the silicon substrate can be made invulnerable to the thermal donor phenomena. Even in a case that a p type silicon substrate of high resistivity is used, the conversion of the conduction type of the p type silicon substrate to n type can be prevented. Thus, according to the present invention, the conduction type of the p type silicon substrate is prevented from converting to n type, and the semiconductor device can include the inductor of high Q.
According to the present invention, because of the polysilicon film formed on the underside of the silicon substrate, which functions as the gettering site, even in a case that the silicon substrate has a low oxygen concentration, metal impurities can be trapped. Thus, according to the present invention, the semiconductor device can prevent characteristic from being degraded even in a case that the silicon substrate has a low oxygen concentration.
According to the present invention, because a highly resistive layer is buried in the silicon substrate, the inductor of high Q can be formed by simple steps, which much contributes to lower costs of the semiconductor device.
According to the present invention, the epitaxial layer is formed on the silicon substrate, and the p-channel MOSFET and the n-channel MOSFET a: e formed on the epitaxial layer, whereby the device isolation can be easily performed without forming a channel stop region.